while trying to use the multiple pipelines feature with Logstash 6.4.1, we ran into multiple problems.
It clearly states in the documentation, that any configuration parameter will disable loading the pipelines.yml file. However, in some real world scenarios, you have to configure the location for the path.settings directory or in any case where the pipelines.yml is located. This again disables that the pipelines.yml file is being loaded.
Conclusion: The feature is not usable right now, because it requires the pipelines.yml file to be located at a default directory, which in our case, does not exist.
Hopefully, this is fixed very soon.